This invention relates to systems and methods used to determine whether a correct pre-load is, or has been, applied to threaded fasteners such as nuts, bolts, and machine screws. More specifically, the invention relates to these systems and methods as applied to pressure-containing equipment like that used in the oil and gas industry.
For the correct and safe on-going operation of equipment, particularly pressure-containing equipment, it is important that the correct pre-load is applied to fasteners and that the load is consistent thereafter.
Traditional techniques of determining fastener pre-load typically rely on measuring the torque applied to the fastener when the correct pre-load is achieved. (Or, turning a nut through a specified angle that stretches the bolt the desired amount). However, differences in tolerances, lubrication, and friction, might require different torques be applied to similar fasteners in a fastening application to achieve the same preload among all of the fasteners. This is a significant limitation of traditional techniques and can lead to highly inconsistent forces, for example, around the circumference of a bolted flange.
Ultimately these differential forces can lead to failures, pressure escapes and fasteners coming apart. These situations can result in environmental damage and harm to personnel.
Prior art systems and methods modify the fastener itself and generally make use of ultrasonic techniques. For example, U.S. Pat. No. 4,899,591 to Kibblewhite discloses an ultrasonic transducer made of a thin piezoelectric sensor consisting of a piezoelectric film sandwiched between two thin electrodes. The transducer is permanently mechanically and acoustically coupled to a head of a fastener and is used to determine the length, tensile load, stress, or other tensile load-dependent characteristics of the member by ultrasonic techniques.
U.S. Pat. No. 7,412,898 B1 to Smith et al. makes use of a fastener having a bore that includes a grounding pin and a radio frequency identification (“RFID”) transponder or tag exposed to the bore (see also US 20060022056 A1 to Sakama et al. disclosing a bolt with an RFID tag and chip for storing information and an antenna for transmitting wirelessly the information stored on the IC chip). The transponder generates a response signal with a unique identifier in response to a radio frequency signal from a transmitter. The grounding pin contacts and grounds the transponder to block transmittal of the response signal when a tensile load applied to the fastener is less than a minimum tensile load value.
U.S. Pat. No. 8,683,869 B2 to Herley et al. discloses a system and method of monitoring fastener pre-load using an ultrasonic transducer on the fastener and electronically transmitting the measured pre-load to a monitoring station for analysis (see also U.S. Pat. No. 7,698,949 B2 to Akdeniz et al. incorporating ultrasonic transducers with load bearing washers). The transducer, which is mounted to the head or threaded end of the fastener by an adhesive, is excited by a voltage pulse and the “time of flight” of the ultrasonic wave is used to indicate pre-load (relative to zero load time of flight). A reader or cap can be placed over the head of the fastener to engage the electrical contacts of the transducer and communicate via an electrical lead to the monitoring station. Alternatively, a radio frequency identification (“RFID”) tag can be mounted on the fastener and operably connected to the transducer.
U.S. Pat. No. 6,378,384 B1 to Atkinson et al. discloses a force sensing device in the form of a washer that includes force sensitive resistors fabricated as thick film resistors which are printed and fired onto an electrically insulated substrate material. The washer is used to monitor pressure pulses of an underlying process or operation that a fastener is in communication with, not fastener pre-load. Others have experimented with ceramic washer designs that make use of piezoelectric filaments connected to a handheld device.